That enormous tapping sound you just heard was millions of comic book fans simultaneously racing to download Marvel's latest iOS app, which promises to bring Spotify-style marketing to the comic book industry at long last.

Marvel Digital Comics Unlimited isn't a new concept -- the company has offered an annual subscription on the desktop for years, which allowed comic book fans to read as many issues as they want for one low price. The problem: Who wants to be tied to their computer all day reading comic books?

After recently dipping their toes into the mobile pond with an HTML5-based version of the Unlimited service that worked pretty well on the iPad, Marvel has now taken the wraps off a full-fledged iOS app that brings seven decades of comics in one tidy place. (Note: Marvel Unlimited is a separate app from the company's existing Marvel Comics app, which offers individual issues for sale only.)

Like music subscription services such as Spotify, Marvel Unlimited also allows offline downloading of comic books, although the app is currently limited to six issues at a time. When connected to a Wi-Fi or cellular network, comic lovers can indulge in all they'd like, streamed right to their device.Marvel Unlimited 1.0 is now available as a free universal app; the 30.9MB download is compatible with all devices running iOS 5.1 or later. A subscription is required, but the app offers a number of free issues as well as previews of everything being offered.